1. Field of the Invention
The present invention relates to a fuel injection control apparatus for an internal combustion engine, and more particularly to a fuel injection control apparatus preferably used in a fuel injection type internal combustion engine equipped with a fuel injection valve supplying atomized liquid fuel such as gasoline from an injection nozzle thereof.
2. Prior Art
A fuel injection method, injecting atomized liquid fuel such as gasoline into an intake passage of an internal combustion engine, has useful and excellent capability of accurately controlling the fuel supply amount in accordance with driving or operational conditions of the internal combustion engine, thereby adjusting the fuel supply amount to a value optimizing output power performance or minimizing harmful emission in the exhaust gas of the engine. To realize such an accurate control of the fuel injection amount supplied to an internal combustion engine, some prior art technologies propose to execute the fuel injection taking account of fuel behavior including adhesion of liquid fuel to the wall of an intake passage. (For example, refer to Laid-open Japanese Patent Application No. SHO 56-47638/1981, Laid-open Japanese Patent Application No. SHO 58-8238/1983, and Laid-open Japanese Patent Application No. HEI 4-153535/1992).
FIGS. 19A through 19D illustrate typical behavior of fuel in a low-temperature engine starting condition, wherein fuel is first injected from an injection valve into an intake passage and then flows into a combustion chamber. FIGS. 19A through 19D are views showing intake, compression, explosion and exhaust strokes of a four-cycle internal combustion engine, respectively. Liquid-state fuel injected from a fuel injection valve 51 into an intake passage 52 is divided into three, a vapor component vaporized immediately after injection, a small particle size liquid component not adhering on the wall of the intake passage 52 and a large particle size liquid component adhering on the wall of the intake passage 52. Both the vapor component and the small particle size liquid component are introduced into a combustion chamber 53 during an intake stroke first taken place after the fuel injection. On the other hand, the large particle size liquid component adhering on the wall of the intake passage 52 remains for a while on the wall without being introduced into the combustion chamber and flows on the wall of the intake passage 52 toward the combustion chamber 53 as liquid film, later entering into the combustion chamber 53 with delay of several strokes.
In short, injected fuel is roughly divided into two, a component introduced into the combustion chamber without delay during an intake stroke immediately after the injection is finished and the other component introduced into the combustion chamber with some delay. A conventional fuel injection control apparatus calculates a fuel injection amount required for obtaining a target air-fuel ratio on the basis of an atomized fuel model representing atomized condition of fuel in the intake passage 52 and a wall flow model representing liquid-film fuel flow along the wall of the intake passage 52.
However, the above conventional fuel injection control apparatus is not perfect in that no consideration is given to the fuel behavior in the combustion chamber 53 after the fuel is once introduced in the combustion chamber 53. Thus, this conventional fuel injection control technology will encounter with a problem of receiving adverse effect of liquid-state fuel residing in the combustion chamber 53, resulting in the failure in the optimization of the engine driving or operational condition.
More specifically, as shown in FIGS. 19A-19D, a combustion chamber of low temperature tends to allow fuel entering from the intake passage 52 to reside as wall wet therein without being vaporized. The wall wet is vaporized to a certain extent in the succeeding compression stroke. Thus vaporized fuel is burnt together with gaseous fuel introduced from the intake passage 52 to the combustion chamber 53 during the preceding intake stroke. Meanwhile, almost all the liquid-state fuel on the wall is not burnt and resides as wall wet.
In short, the above-described conventional fuel injection control apparatus does not take account of adverse effect caused by the fuel entering as wall wet into the combustion chamber 53 and a vaporized component of the fuel residing as wall wet in the combustion chamber 53. Therefore, it was not possible to control an actual gaseous air-fuel ratio (i.e. a weight ratio of air to gaseous fuel) in the combustion chamber 53 to a desired value. Especially, fuel adhering on the wall in the combustion chamber 53 increases its amount with lowering temperature of the engine. Thus, the actual gaseous air-fuel ratio in the combustion chamber 53 is largely deviated from the desired ratio. If the actual gaseous air-fuel ratio in the combustion chamber 53 is too much lean, the start-up of engine will be delayed. On the other hand, if the actual gaseous air-fuel ratio is too much rich, harmful emission components such as HC will increase.
Even if the engine succeeds to start, too much lean air-fuel ratio will later cause a problem of suffering lack of torque when acceleration is required. More specifically, when an amount of wall wet is small in the combustion chamber 53, the actual gaseous air-fuel Patio is maintained within a predetermined range in the combustion chamber 53, thus assuring a firing for combustion. On the contrary, if an amount of wall wet increases in the combustion chamber 53, the actual gaseous air-fuel ratio will be deviated out of the predetermined range, thus inducing failure of firing which results in knocking with deterioration of drivability.